Density functional theory calculations implemented by the SIESTA code are used to study the interactions of the saccharides N-acetylgalactosamine (GalNAc) and glucuronic acid (GlcA) with the (0001) and (01 (1) over bar0) surfaces of the mineral hydroxyapatite (HAP). GalNAc and GlcA are the constituent monosaccharides of chondroitin, which is a glycosaminoglycan found in bone and cartilage, and whose interactions with HAP have been implicated as a controlling factor in the process of biomineralization. Geometry optimization calculations are used to identify low-energy adsorption structures of the monosaccharides on the HAP surfaces, and to calculate the corresponding adsorption energies. The calculations show that GalNAc interacts with HAP principally through its hydroxy and acetyl amine functional groups, and deprotonated GlcA interacts principally through its hydroxy and carboxylate functional groups. The mode and the strength of adsorption depend on the orientation of the saccharide with respect to the HAP surface, which has implications for the structural conformation of chondroitin chains in the presence of HAP. Both monosaccharides bind more strongly to the (01 (1) over bar0) surface than to the (0001) surface.